The filmmaker heaped praise on the Korean star, who plays deadly assassin Raizo in the martial arts-action film.

"He's an incredible star in Asia so the discipline and the presence comes as a package already, so you want to take advantage of that, then Rain just kept pushing it to another level for us - it was pretty great actually," he said.

"Working with him on Speed Racer, seeing his physicality, recognising his screen presence, and then knowing if we were going to do this ninja movie that he'd be the perfect person for it."

James was in awe of Rain's rigorous training schedule.

"His training regime was incredible - you know like five or six days, eight hours a day, they start the morning with this intense physical training, then into action choreography in the afternoon, then more training at the end of the day," he recalled.

"I would go and watch him train for five minutes and then get tired and have to go away, knowing he had another seven hours and 55 minutes to go."